1. Field of the Invention
The present invention relates generally to an automatic control system for use in a temperature control system for a semiconductor equipment, and an automatic control method using the same. More specifically, the invention relates to an automatic control system having an improved optimum regulator, and an automatic control method using the same.
2. Description of the Prior Art
In general, in order to fabricate a semiconductor device such as an integrated circuit, it is required to repeatedly carry out various treatments, such as a film forming treatment at elevated temperatures, an oxidation and diffusion treatment and an etching treatment, with respect to an object to be processed, such as a semiconductor wafer.
For example, in the case of a sheet-type thermal treatment equipment, a semiconductor wafer mounted on a mounting table in a processing container is heated to a process temperature, e.g., a temperature of about 500.degree. to 600.degree. C., by heating means, e.g., a heating lamp, to carry out predetermined treatments, e.g., a film forming treatment and an oxidation treatment.
Thus, when a semiconductor wafer is treated at elevated temperatures, it is important whether the temperature of the wafer can be accurately maintained at a process temperature in order to ensure the uniformity of thermal treatment, and how the temperature of the wafer can be risen to the process temperature in order to obtain a high throughput and whether the process temperature can be maintained without causing overshoot.
With respect to the temperature control of a wafer, in place of a classical control theory wherein a control system is designed on the basis of frequency characteristics around a single input and a single output, a modern control theory suitable for a complicated system, such as a digital control of a system having multiple inputs and multiple outputs, has been used (e.g., see "Mechanical System Control"; Mar. 20, 1984; Ohm).
In this modern control theory, studies of optimum controllability and stability for controlling so as to minimize a performance function in a given specification have been actively made, and the state expression, which express the input-output relationship as well as the internal state, is used as the expression of the system.
In a method for designing a control system on the basis of an equation of state which expresses a state expression as an equation, a regulator for stabilizing a closed loop system and for improving the excessive characteristics thereof has been used. In this case, it is hardly possible to directly measure all the state variables. In this case, a state observer is used.
A typical automatic control system for use in a temperature control system for a semiconductor equipment will be described. FIG. 7 is a functional block diagram of an example of a typical automatic control system for use in a temperature control system for a semiconductor equipment.
In the diagram, reference number 2 denotes an object to be controlled, such as a wafer in a semiconductor equipment, and the temperature of this object is controlled. Reference number 4 denotes an optimum regulator for determining a matrix of feedback coefficients so as to minimize an appropriate performance function and for finding a compromise point of necessary equations. Reference numbers 8 and 10 denote an integrator gain and an integrator, respectively.
In addition, reference number 12 denotes a state stabilizer gain, and reference number 14 denotes a state observer. In the state feedback control, this system is a most basic system since the state is a minimum amount of information for determining the behavior of the system. In this system, the value of a state variable can not be directly measured according to circumstance. In such a case, the value of the state variable is presumed by means of the state observer 14 on the basis of the output of the object to be controlled, the output being able to be directly measured.
The temperature obtained via a thermocouple from the object 2 to be controlled, i.e., a controlled variable, is returned to an adder 6 and added therein so as to have a negative sign, so that the difference between the obtained temperature and a set temperature serving as a target value is derived. This difference is tempered by a gain of the integrator gain 8, and integrated by the integrator 10 to derive a manipulated variable.
On the other hand, a state variable, which can not be measured, is derived by means of the state observer 14 on the basis of the measured controlled variable. The derived state variable and the measured state variable are tempered by a gain of the state stabilizer gain 12 to derive a manipulated variable. The manipulated variable thus derived is added, in an adder 16, to the manipulated variable derived in the integrator 10, so that the temperature of the object 2 is controlled on the basis of the added amount. Furthermore, all of these operations are carried out by means of a software using a microcomputer or the like.
In a control system of this type, a single gain is generally set. In addition, most of actual systems are non-linear systems. In these systems, it is required to stably respond to all the control region, so that a stably operable maintenance gain is must be selected as a set gain.
That is, in a control region wherein the rate of state variation of the control system is higher than that of the control region, since there is an unstable factor for the control, a gain must be set so as not to immediately respond to a very fast change of state.
Therefore, it takes a lot of time until the controlled variable of the object to be controlled is stabilized after approaching a target value, so that there is a problem in that a sufficiently fast control can not be carried out.
In addition, in order to eliminate the aforementioned problem, if separate gains are provided for the respective temperature zones similar to conventional systems, a great number of parameters (state variable.times.temperature zone+.alpha.) must be provided, so that there is a problem in that the system has a complicated design.